1. Field of the Invention
The present invention relates to a scanning optical apparatus having a rotary polygon mirror for scanning a light beam such as a laser beam on a photosensitive member.
2. Related Background Art
In these years, the scanning optical apparatus of this type is required to rotate the polygon mirror at high speed or at high accuracy, and particularly, laser beam printer apparatus use a bearing rotating apparatus rotating in non-contact in order to achieve a high-accuracy deflection-scanning apparatus.
In the bearing rotating apparatus for rotating the rotary polygon mirror used in the conventional scanning optical apparatus, as shown in FIG. 1, a stationary shaft 2 of a ceramic material is standing from a housing 1 mounted on a frame or the like, a rotary sleeve 4 made of a ceramic material with a drive magnet 3 attached thereto is rotatably fit over the stationary shaft 2, and a rotary polygon mirror 6 is fixed by a spring 5 to the rotary sleeve 4. On the housing 1 there is a base 8 to which a stator 7 is mounted at a position opposite to the drive magnet 3, thus constituting a drive motor for rotating the rotary sleeve 4.
Further, a permanent magnet 9 is attached to the lower end of the rotary sleeve 4, and a second permanent magnet 10 is fixed to a lower portion of the stationary shaft 2 so as to be vertically opposed to the permanent magnet 9 and to be repulsive thereto, thus supporting a load in the thrust direction by repulsion between the permanent magnets 9, 10.
Once the driving motor is driven, an air membrane is formed between the rotary sleeve 4 and the stationary shaft 2. This air membrane supports the rotary sleeve 4 in the radial direction, and the rotary sleeve 4 and rotary polygon mirror 6 can rotate in non-contact.
Since the thrust bearing was kept floating only by a repulsive force between the pair of permanent magnets in the scanning optical apparatus as in the above-described conventional example, the apparatus, however, had such defects that thrust bearing stiffness was low, that accuracy of height position of the rotary sleeve could not be enhanced so much, and that control of a motion in the thrust direction due to disturbance etc. was difficult.